Rechargeable lithium-ion batteries are attractive energy storage systems for portable electronics and electric and hybrid-electric vehicles because of their high specific energy compared to other electrochemical energy storage devices. A typical Li-ion cell contains a negative electrode, a positive electrode, and a separator region between the negative and positive electrodes. Both electrodes contain active materials that insert or react with lithium reversibly. In some cases the negative electrode may include lithium metal, which can be electrochemically dissolved and deposited reversibly. The separator contains an electrolyte with a lithium cation, and serves as a physical barrier between the electrodes such that none of the electrodes are electronically connected within the cell.
Typically, during charging, there is generation of electrons at the positive electrode and consumption of an equal amount of electrons at the negative electrode, and these electrons are transferred via an external circuit. In the ideal charging of the cell, these electrons are generated at the positive electrode because there is extraction via oxidation of lithium ions from the active material of the positive electrode, and the electrons are consumed at the negative electrode because there is reduction of lithium ions into the active material of the negative electrode. During discharging, the exact opposite reactions occur.
In particular, batteries with a lithium metal negative electrode afford exceptionally high specific energy (in Wh/kg) and energy density (in Wh/L) compared to batteries with conventional carbonaceous negative electrodes. Other metals, such as Zn, Al, Mg, Si, and others, also have a favorable specific energy and energy density. However, the cycle life of such systems is rather limited due to (a) formation of dendrites during recharge that may penetrate the separator and short the cell and/or result in fragmentation and capacity loss of the negative electrode; (b) morphology changes in the metal upon extended cycling that result in a large overall volume change in the cell; and (c) changes in the structure and composition of the passivating layer that forms at the surface of the metal when exposed to certain electrolytes, which may isolate some metal and/or increase the resistance of the cell over time.
When high-specific-capacity negative electrodes such as a metal are used in a battery, the maximum benefit of the capacity increase over conventional systems is realized when a high-capacity positive electrode active material is also used. For example, conventional lithium-intercalating oxides (e.g., LiCoO2, LiNi0.8Co0.15Al0.05O2, Li1.1Ni0.3Co0.3Mn0.3O2) are typically limited to a theoretical capacity of ˜280 mAh/g (based on the mass of the lithiated oxide) and a practical capacity of 180 to 250 mAh/g, which is quite low compared to the specific capacity of lithium metal, 3863 mAh/g. The highest theoretical capacity achievable for a lithium-ion positive electrode is 1794 mAh/g (based on the mass of the lithiated material) for Li2O. Other high-capacity materials include BiF3 (303 mAh/g, lithiated), FeF3 (712 mAh/g, lithiated), and others. See G. G. Amatucci and N. Pereira, “Fluoride based electrode materials for advanced energy storage devices.” Journal of Fluorine Chemistry, 2007. 128(4): p. 243-262 and J. Cabana, L. Monconduit, D. Larcher and M. R. Palacin, “Beyond Intercalation-Based Li-Ion Batteries: The State of the Art and Challenges of Electrode Materials Reacting Through Conversion Reactions.” Advanced Materials, 2010. 22(35): p. E170-E192. Unfortunately, all of these materials react with lithium at a lower voltage compared to conventional oxide positive electrodes, hence limiting the theoretical specific energy; however, the theoretical specific energies are still very high (>800 Wh/kg, compared to a maximum of ˜500 Wh/kg for a cell with lithium negative and conventional oxide positive electrodes).
To enable electric vehicles with a range approaching that of present-day vehicles (>300 miles) a battery chemistry with a significantly higher specific energy is required than the present state of the art Lithium-ion batteries. FIG. 1 depicts a chart 10 showing the range achievable for a vehicle using battery packs of different specific energies versus the weight of the battery pack. In the chart 10, the specific energies are for an entire cell, including cell packaging weight, assuming a 50% weight increase for forming a battery pack from a particular set of cells. The vertical line 12 gives the maximum acceptable battery pack weight, according to the goals set by the Department of Energy. As indicted by line 12, the U.S. Department of Energy has established a weight limit of 200 kg for a battery pack that is located within a vehicle. Accordingly, only a battery pack with about 600 Wh/kg or more can achieve a range of 300 miles.
Various lithium-based chemistries have been investigated for use in various applications including in vehicles. FIG. 2 depicts a chart 20 which identifies the specific energy and energy density of various lithium-based chemistries. In the chart 20, only the weight of the active materials, current collectors, binders, separator, and other inert material of the battery cells are included. The packaging weight, such as tabs, the cell can, etc., are not included. As is evident from the chart 20, through the use of a lithium metal negative electrode and a positive electrode reacting the oxygen from air, a driving range above 300 miles is possible.
A typical lithium/air electrochemical cell 50 is depicted in FIG. 3. The cell 50 includes a negative electrode 52, a positive electrode 54, and a porous separator 56. The negative electrode 52 is typically metallic lithium. The positive electrode 54 includes electrode particles such as particles 60 possibly coated in a catalyst material (such as Au or Pt) and suspended in a porous, electrically conductive matrix 62. An electrolyte solution 64 containing a salt such as LiPF6 dissolved in an organic solvent such as dimethyl ether or CH3CN permeates both the porous separator 56 and the positive electrode 54. The LiPF6 provides the electrolyte with an adequate conductivity which reduces the internal electrical resistance of the cell 50 to allow a high power.
A portion of the positive electrode 52 is enclosed by a barrier 66. The barrier 66 in FIG. 3 is configured to allow oxygen from an external source 68 to enter the positive electrode 54. The wetting properties of the positive electrode 54 prevent the electrolyte 64 from leaking out of the positive electrode 54, or another electrolyte-containment method is used. Oxygen from the external source 68 enters the positive electrode 54 through the barrier 66 while the cell 50 discharges and oxygen exits the positive electrode 54 through the barrier 66 as the cell 50 is charged. The barrier 66 may also supply oxygen for all of the cells in a stack, and therefore not be directly adjacent to individual cells.
As an example of the reactions and technological requirements in a metal/air cell, during discharge of the lithium/air cell, Li metal dissolved from the negative electrode 52, while at the positive electrode 54, Li+ ions in the electrolyte react with oxygen and electrons to form a solid Li2O2 (or Li2O) product, which may coat or fill the conductive matrix of the positive electrode 54 and/or fill the pores of the electrode. The solid product is thought to be electronically insulating, at least in its crystalline, bulk form. During charge of the cell, the Li2O2 (or Li2O) is oxidized to form O2, Li+ in the electrolyte, and electrons at the positive electrode 54, while at the negative electrode 52, Li+ in the electrolyte is reduced to form Li metal. The reactions that may occur at each electrode are shown by the following (only the reaction forming Li2O2 is shown here):
      Li    ↔                  Li        +            +                        e          -                ⁢                                  ⁢                  (                      negative            ⁢                                                  ⁢            electrode                    )                                        1        2            ⁢              O        2              +          2      ⁢                          ⁢              Li        +              +                            e          -                ⁢                  ⟷          catalyst                ⁢                  Li          2                    ⁢      O      ⁢                          ⁢              (                  positive          ⁢                                          ⁢          electrode                )                        O      2        +          2      ⁢                          ⁢              Li        +              +                            e          -                ⁢                  ⟷          catalyst                ⁢                  Li          2                    ⁢                        O          ⁢                                                2            ⁢                          ⁢              (                  positive          ⁢                                          ⁢          electrode                )            
There are significant challenges that must be addressed for the lithium-air system to become commercially viable. Important challenges include reducing the hysteresis between the charge and discharge voltages (which limits the round-trip energy efficiency), improving the number of cycles over which the system can be cycled reversibly, limiting dendrite formation at the lithium metal surface, protecting the lithium metal (and possibly other materials) from moisture and other potentially harmful components of air, and designing a system that actually achieves a high specific energy and has an acceptable specific power. FIG. 4(a) shows a typical discharge and charge curve for a lithium/air system. As can be seen in FIG. 4(a), the discharge voltage 70 (approximately 2.5 to 3 V vs. Li/Li+) is much lower than the charge voltage 72 (approximately 4 to 4.5 V vs. Li/Li+). The equilibrium voltage (or open-circuit potential) of the lithium/air system is approximately 3 V. Hence, the voltage hysteresis is not only large, but also very asymmetric. FIG. 4(b) is a plot of the decay in the discharge capacity for both an uncatalyzed (carbon only) 74 and catalyzed (EMD-carbon) design 76 over a number of cycles. The experimental results shown in FIGS. 4(a) and 4(b) demonstrate two principle problems with the lithium/air system: the large voltage hysteresis between the charge and discharge curves and rapid loss of capacity with cycling.
Abraham and Jiang published one of the earliest papers on the “lithium-air” system. See Abraham, K. M. and Z. Jiang, “A polymer electrolyte-based rechargeable lithium/oxygen battery”; Journal of the Electrochemical Society, 1996. 143(1): p. 1-5. Abraham and Jiang used an organic electrolyte and a positive electrode with an electrically conductive carbon matrix containing a catalyst to aid with the reduction and oxidation reactions. Previous lithium-air systems using an aqueous electrolyte have also been considered, but without protection of the Li metal anode, rapid hydrogen evolution occurs. See Zheng, J., et al., “Theoretical Energy Density of Li—Air Batteries”; Journal of the Electrochemical Society, 2008. 155: p. A432.
What is needed is a battery which improves the mass-transport effects within the cell, including the cathode. A battery which exhibits improved mass-transport effects during both charge and discharge would be further beneficial.